Exploiting the heat generated by our streets

Benjamin Gillies, Troy Media07.15.2012

On a steamy July or August day, asphalt can become as hot as 60 degrees Celsius – and stay that way well after sunset. This pushes up the temperature of cities covered in the black material, increasing energy demand, air conditioning costs, air pollution and even heat-related illness and mortality.

WINNIPEG – Watching the rays radiate off the pavement on a scorching summer afternoon, it is obvious that asphalt, common in road and sidewalk construction, is incredibly good at trapping heat.

In fact, on a steamy July or August day it can become as hot as 60 degrees Celsius – and stay that way well after sunset. This pushes up the temperature of cities covered in the black material, increasing energy demand, air conditioning costs, air pollution and even heat-related illness and mortality.

Engineers have long recognized this problem, known as the heat island effect, and much effort has gone into mitigating its unpleasant results. It has been recommended cities paint all black surfaces white, for example, or plant more trees.

Recently, however, experts at the Worcester Polytechnic Institute in Massachusetts and the University of Rhode Island have suggested we adopt a different approach, viewing the heat-absorbent capacity of asphalt not as a burden, but an untapped asset.

While we do not often think of it this way, when asphalt heats up, what it is doing – very effectively – is collecting solar energy. Considering the massive surface area covered by roads and parking lots in the average North American city, this is a huge amount of free power that is right now being lost. As such, the optimal approach to dealing with overheated asphalt may not be to fight its heat-retaining power, but instead attempt to capture it for productive use.

Since 2010, researchers at both schools have been experimenting with different ways of harnessing this solar energy supply, such as laying a series of pipes beneath the street surface. These pipes could eventually be used to heat water that is then pumped to surrounding buildings, or filled with other liquids that turn to vapour used to drive steam turbines generating electricity to power street lamps and for other uses.

Employing similar heat pumps and heat storage facilities, engineers in Spain are also working on using the trapped heat to maintain the temperature of roads above freezing levels in winter, preventing the formation of ice on the surface. While not only improving traffic safety, such a breakthrough would mean less salt was required to reduce frost, and the cost of street maintenance could be reduced as the extreme temperature changes that cause cracking would be mitigated.

Asphalt is cheap, and with cash-strapped cities already struggling to pay their bills, they may be hard-pressed to justify spending money on more costly alternative paving materials. As such, capturing the heat generated by asphalt may ultimately be the most realistic solution to this challenge. It may actually be the most sustainable approach as well. After all, asphalt is recyclable, making it a relatively environmentally friendly product.

By some estimates, pavement covers up to half of North America’s urban area. That is a huge amount of space taken up for just one purpose: transportation. By underlying our road network with energy-harnessing infrastructure, suddenly our streets and parking lots would serve a dual transportation/energy-production function – far more efficient than taking up additional space laying solar panels or building another generating station on the outskirts of town. Finally, unlike other solar energy systems that primarily run only during the day, asphalt is an incredible insulator, meaning it can continue providing useful power even after sunset.

With Canadian summers becoming longer and hotter, coming up with new and affordable measures for cooling down our cities and saving energy will become more critical. It remains to be seen whether these types of systems are feasible and cost-effective, but further research is worthwhile.

Done successfully, harnessing this power could simultaneously cool off the street surface and send the heat somewhere useful. The heat island effect would be reduced; the cooler pavement would have a longer lifespan; and the free and clean solar energy would be going to beneficial activities, reducing the need to build power sources elsewhere. In the end, the best way to beat the heat may be to get it working for us.

Troy Media columnist Benjamin Gillies is a political economy graduate from the University of Manitoba, where he focused on urban development and energy policy. He works as an urban development consultant in Winnipeg.

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